I am trying this out ----
I'm using selectors in <div>s to format paragraphs, so that I don't have to use many styles for lots of paragraphs. This can keep all the styling in the div, and feels much simpler. I am only using this method for short segments.
For example, for some poetry that has altenating lines with deeper margins/indents:

i tried this out on ADE and the sony desktop reader as well as PRST1 and it works across the board there. iBooks has no problem, as well as another 7 or so reader apps on the iPad. the iOS reader app i've come across so far that doesn't register the + + + + is Kobo app for ios.

i'd be interested to know if it's supported on the actual Kobo, though

i tried this out on ADE and the sony desktop reader as well as PRST1 and it works across the board there. iBooks has no problem, as well as another 7 or so reader apps on the iPad. the iOS reader app i've come across so far that doesn't register the + + + + is Kobo app for ios.

i'd be interested to know if it's supported on the actual Kobo, though

No sooner seen than done.

I have it open on my Kobo Touch right now. The three sizes of text are visible, at increasing sizes, but the lines of text in each of the 3em and 4em levels run together - the top of the 4 in 4em reaches almost as high as the cross bar in the e in 'here's.'

Nope. That's why I specifically mentioned ePubs in my reply. However it does work with KF8 enabled devices and apps and many other Webkit based readers.

Doitsu:

I did read your reply, and you did indeed specifically say ePUBs and KF8, but of course, Amazon keeps swearing that iOS is "gunna be" KF8 any second, which is why I asked--plus, there's no reason for me to think that everything we do is magically always right. (I wish). You might have had something up your sleeve! ;-)

{sigh}. I'd love to use the selectors. But freaking K4iOS.... just making sure it wasn't us.

For some time I've used the +++siblings for front pages, where font size and formatting varies from line to line. I've also found it handy on several other occasions.
I've been sorely tempted to drop the <p> tag altogether, and just use formatted <div>'s for paragraphs instead. It's so often I want to include some kind of block-style element inside a paragraph, only to yet again experience the epub/html validator sending me dirty looks...